Fractures account for 25% of all musculoskeletal injuries in the U.S., leading to 8 million treatment visits per year and 60 million lost work days. Rates of healing complication (delayed or non-union) range from 1-10%, affecting more than 100,000 patients annually. Stress fractures are a repetitive use injury affecting 500,000 people each year, mostly athletes and military recruits. Recently, a rare but devastating type of stress fracture - the atypical femur fracture - has been described, linked to bisphosphonate treatment. Although the methods by which full fractures and stress fractures heal are distinct, in both cases the key to achieving a functional repair is robust, periosteal bone formation (osteogenesis). The long-term goal of this project is to understand the mechanobiological pathways that regulate osteogenesis in the context of bone repair of the adult skeleton. Angiogenesis and inflammation are two processes integral to osteogenesis during bone repair, and recent evidence from the past funding period suggests that the osteocyte may play a role in initiating or regulating these processes. The overall hypothesis of this proposal is that the osteocyte contributes angiogenic and inflammatory factors critical to bone repair. Murine models for full fracture (closed, stabilizd femoral shaft fracture) and stress fracture (ulnar fatigue loading) will be used to create reproducible bone injuries. Histology, microCT and qPCR gene expression will be used to assess inflammatory, angiogenic and osteogenic outcomes. In Aim 1, the role of the prototypic pro-angiogenic factor VEGFA (vascular endothelial growth factor) will be examined in different cell types (osteocyte, osteoblast, endothelial cell) using Cre-loxP methods. The recent availability of inducible Cre drivers makes this aim feasible in adult mice. In Aim 2, the role of factors classically related to inflammation, but also relevant to vascular responses and angiogenesis, will be evaluated. Interleukin 6 (IL-6) and inducible nitric oxide synthase (iNOS) are expressed by bone cells and immune cells. In the past funding period iNOS was shown to mediate increased blood flow and osteogenesis after stress fracture. Using Cre-loxP methods and radiation chimera, the relative importance of osteocytes and immune cells in producing these two factors will be examined. In summary, the project will determine whether the osteocyte, already recognized as a master regulator of bone homeostasis and remodeling, plays an important role in bone repair. Better understanding of endogenous repair mechanisms may inform future treatment strategies to augment bone healing.